The present invention relates to an ion conductive polymer electrolyte membrane which is used for a solid polymer type fuel cell. Such an ion conductive polymer electrolyte membrane allows permeation of hydrogen ions generated at a fuel electrode (anode) to an air electrode (cathode).
With development of electronic instruments, fuel cells have recently been attracting attention as small portable power sources with high power density. The fuel cells, unlike conventional power generation systems, produce electric energy by electrochemically reacting fuel, such as hydrogen or methanol, with oxygen in air. The fuel cells are classified into, for example, a solid polymer type, a solid oxide type, a molten carbonate type, and a phosphoric acid type, in accordance with types of electrolyte. Of those, a solid polymer type fuel cell has been studied vigorously as a portable power source due to operability at normal temperatures (100° C. or below), a short starting time, and easy reduction in size.
The solid polymer type fuel cell includes a fuel electrode, an air electrode, and a polymer electrolyte membrane such as an ion conductive polymer electrolyte membrane provided therebetween. In the solid polymer type fuel cell, hydrogen as a fuel supplied from a side of the fuel electrode dissociates into hydrogen ions and electrons. Then, the hydrogen ions permeate through the polymer electrolyte membrane, reach the air electrode, and react with oxygen in air, to thereby generate water. Through a series of those reactions, electric energy is produced.
A fluorine-based polymer electrolyte membrane having an acidic group such as perfluorocarbon sulfonic acid introduced thereinto has been used extensively as this type of polymer electrolyte membrane. For example, Japanese Laid-Open Patent Publications No. 05-255522 and No. 06-251780 each disclose a solid polymer type fuel cell employing NAFION (trade name) as perfluorocarbon sulfonic acid. Further, there is proposed an electrolyte membrane having a strong acid group introduced into an aromatic-containing polymer. For example, Japanese National Phase Laid-Open Patent Publication No. 2002-533890 and Japanese Laid-Open Patent Publication No. 2003-288916 each disclose a polymer electrolyte membrane having a strong acid group such as a sulfonic group introduced into a polymer having chemical resistance and thermal resistance such as polysulfone or polybenzimidazole.
Meanwhile, polymer electrolyte membranes having structures disclosed in the following patent documents are known as ion conductive polymer electrolyte membranes having anisotropy, for example. Japanese Laid-Open Patent Publication No. 2003-234015 discloses an ion conductive polymer electrolyte membrane composed of a polymer such as polyacrylic acid having an ionic dissociative group such as a carboxyl group. In this ion conductive polymer electrolyte membrane, said polymer is orientated by applying an electric field thereto. Japanese Laid-Open Patent Publication No. 08-165360 discloses an electrically conductive polymer orientated membrane composed of a conductive polymer derivative such as water-soluble polypyrrole. In the membrane, a conductive polymer derivative is orientated in a thickness direction of the membrane by applying a magnetic field thereto.
However, perfluorocarbon sulfonic acid used in each of Japanese Laid-Open Patent Publications No. 05-255522 and No. 06-251780 has a low glass transition temperature and barely retains moisture at about 100° C., which is an upper limit for the operation temperature of the solid polymer type fuel cell. Thus, sufficient ionic conductivity cannot be exhibited. Further, the polymers used in each of Japanese National Phase Laid-Open Patent Publication No. 2002-533890 and Japanese Laid-Open Patent Publication No. 2003-288916 have stiff molecular chains that are not controlled with respect to their orientation. Thus, sufficient ionic conductivity cannot be exhibited in a thickness direction of the polymer electrolyte membrane.
The ion conductive polymer electrolyte membrane disclosed in Japanese Laid-Open Patent Publication No. 2003-234015 has large pores each with a pore size of around 10 μm or more, to thereby cause a so-called crossover phenomenon. Thus, a fuel may be lost due to the crossover phenomenon during power generation to reduce power generation capacity, or a side reaction with air may significantly reduce the power generation capacity. Furthermore, the ion conductive polymer electrolyte membrane has low thermal resistance due to denaturation of the polymer at the upper limit of operation temperature. Thus, the ion conductive polymer electrolyte membrane is not suitable for the solid polymer type fuel cell.
The electrically conductive polymer orientated membrane disclosed in Japanese Laid-Open Patent Publication No. 08-165360 is aimed at arbitrary control of the direction of electron conduction. In this membrane, electrons together with hydrogen ions permeate through the membrane, to provide substantially no voltage difference between a fuel electrode and an air electrode. Thus, the electrically conductive polymer orientated membrane is not suitable for the solid polymer type fuel cell.
The present invention has been made in view of the problems present in conventional technology. An objective of the present invention is therefore to provide an ion conductive polymer electrolyte membrane which has excellent thermal resistance and high ionic conductivity in a thickness direction, and thus which is suitable for a solid polymer type fuel cell. Another objective of the present invention is to provide a production method for an ion conductive polymer electrolyte membrane allowing easy production of the ion conductive polymer electrolyte membrane.